Because sonically all these discrete R2R ladder dacs are very close to each other and all have well over >2v output (most amps only need <1v for full ouput) and low impedance outputs, as strong as any preamp.
I would then start looking at the one that has an inbuilt volume control to go direct with, as best best preamp is no preamp, and you’ll save a stack of money also.
My MSB discrete R2R ladder dac sounds stunning going direct using it’s own volume control, and betters any preamps that have been used even $$$$$K ones friends have tried to challenge it with.
Cheers George